linux-lts: privilege escalation

It was reported that possible use-after-free vulnerability in keyring
facility, possibly leading to local privilege escalation, was found.
Function join_session_keyring in security/keys/process_keys.c holds a
reference to the requested keyring, but if that keyring is the same as
the one being currently used by the process, the kernel wouldn’t
decrease keyring->usage before returning to userspace. The usage field
can be possibly overflowed causing use-after-free on the keyring object.